1. Field of the Invention
The present invention relates to a piezoelectric electro-acoustic transducer for a piezoelectric receiver and a piezoelectric sounder.
2. Description of the Related Art
Piezoelectric electro-acoustic transducers have been widely used for piezoelectric sounders and piezoelectric receivers that produce an alarm sound or an operating sound in electronic instruments, home electric appliances, and portable telephones. In such a piezoelectric electro-acoustic transducer, a transducer has been proposed which improves productivity and acoustic conversion efficiency and which is miniaturized by using a rectangular diaphragm.
In Japanese Unexamined Patent Application Publication No. 2000-310990, a piezoelectric electro-acoustic transducer is disclosed that includes a rectangular piezoelectric diaphragm and a casing having a bottom wall, four sidewalls, a support unit for supporting the diaphragm inside two sidewalls opposing each other, and first and second terminals disposed in the support unit for connecting to the outside, wherein the diaphragm is accommodated within the casing, and two sides of the diaphragm opposing each other are fixed to the support unit via an adhesive or an elastic adhesive while the clearance between the remaining two sides of the diaphragm and the casing is sealed with the elastic adhesive and the diaphragm and the first and second terminals are electrically connected via a conductive adhesive.
The reason for sealing the space between the diaphragm and the casing is to isolate spaces on the top and bottom surfaces of the diaphragm so as to provide acoustic spaces on the top and bottom surfaces of the diaphragm. To minimize the suppression of the vibration of the diaphragm, a soft elastic adhesive, such as a silicone adhesive, is used as the elastic adhesive.
To reduce the frequency, recently, the thickness of the diaphragm has been greatly reduced, and thin diaphragms with a thickness of about several tens to one hundred micrometers are used. With such a thin diaphragm, the effect of the support structure on frequency characteristic is increased.
For example, if the diaphragm is directly connected to the terminals fixed to the casing with a thermo-setting conductive adhesive, the diaphragm is stressed by a curing contraction force of the conductive adhesive, which produces dispersion in frequency characteristics. Also, since a Young's modulus of the conductive adhesive after being cured is relatively large, the vibration of the diaphragm is suppressed and cracks are produced in the conductive adhesive by the vibration of the diaphragm.
Japanese Unexamined Patent Application Publication No. 2003-9286 discloses a piezoelectric electro-acoustic transducer that includes a casing having a support unit for supporting lower surfaces of two or four sides of a piezoelectric diaphragm, terminals having internal connection portions exposed in the vicinities of the support unit, a first elastic adhesive applied between the external periphery of the piezoelectric diaphragm and the internal connection portions of the terminals so as to fix the piezoelectric diaphragm to the casing, a conductive adhesive applied between an electrode of the piezoelectric diaphragm and the internal connection portions of the terminals so as to be spaced from the upper surface of the first elastic adhesive and to electrically connect the electrode of the piezoelectric diaphragm to the internal connection portions of the terminals, and a second elastic adhesive for sealing the clearance between the external periphery of the piezoelectric diaphragm and the internal periphery of the casing.
The first elastic adhesive may be a urethane adhesive, for example, and the second elastic adhesive is a material having a smaller Young's modulus than that of the first elastic adhesive, such as a silicone adhesive.
FIG. 13 shows a connection portion between a piezoelectric diaphragm 30 and a terminal 31 in Japanese Unexamined Patent Application Publication No. 2003-9286. Between the piezoelectric diaphragm 30 and the terminal 31, a first elastic adhesive 32 is applied so as to rise and a conductive adhesive 33 is further applied thereon so as to prevent changes in frequency characteristics of the diaphragm 30 due to a curing contraction stress of the conductive adhesive 33, and to avoid cracks being generated after the conductive adhesive 33 is cured.
However, in this case, a support unit 34 and the piezoelectric diaphragm 30 are bonded by the first elastic adhesive 32, such that the diaphragm 30 is restricted and the vibration thereof is suppressed.
In Japanese Unexamined Patent Application Publication No. 2003-23696, a transducer is disclosed which includes a support unit provided in a casing for supporting four corner lower surfaces of a piezoelectric diaphragm, and between the piezoelectric diaphragm and a terminal, a first elastic adhesive is applied at a location in the vicinity of the support unit and a conductive adhesive is further applied thereon.
FIG. 14 shows a connection portion between the piezoelectric diaphragm 30 and the terminal 31 in Japanese Unexamined Patent Application Publication No. 2003-23696. In this case, since a cavity is provided under the piezoelectric diaphragm 30 in a region where the first elastic adhesive 32 is applied, although it is unlikely that the piezoelectric diaphragm 30 will be restricted by the first elastic adhesive 32, the first elastic adhesive 32 flows downward passing through the clearance between the diaphragm 30 and a casing 35, such that the first elastic adhesive 32 is not raised between the diaphragm 30 and the terminal 31.
The elastic adhesive is typically a cold-setting adhesive and a thermo-setting adhesive. In the cold-setting adhesive, since the viscosity in coating (thixotropy) is relatively large and the curing time is short, the adhesive cannot flow downward passing through the clearance between the diaphragm and the casing. However, the cold-setting adhesive begins to cure during coating which deteriorates work efficiency by the clogging a coating device. The Young's modulus after the adhesive is cured is relatively high such that the cold-setting adhesive restricts the diaphragm.
On the other hand, in the thermo-setting adhesive with a low viscosity (thixotropy), the adhesive does not begin curing during coating such that coating work efficiency is outstanding, and the diaphragm is not restricted because the Young's modulus after being cured is relatively low.
However, if the low-viscosity elastic adhesive is used, the elastic adhesive flows down toward the bottom wall of the casing as described above and the elastic adhesive cannot be raised between the diaphragm and the terminal. Therefore, a restricting force of the conductive adhesive which will be applied and cured thereafter may act on the diaphragm so as to inhibit the vibration.
As described above, with a conventional structure, it is difficult to simultaneously satisfy three conditions: 1) the diaphragm being held without substantial restriction, 2) coating work efficiency of the elastic adhesive is improved, and 3) the elastic adhesive being applied so as to rise.